In glasses of a plastic material, a frame 20 and an earpiece 21 can be connected together by means of a plastic hinge 22, as shown in FIG. 6. The plastic hinge 22 is so constructed that a pivotally supporting portion or support portion 24 formed integrally with the frame 20 at one end thereof can be clamped by a pair of vertically spaced clamp ends 25 formed in the earpiece 21 at one end thereof. The support portion 24 is formed with pivot holes 28 in the top and bottom faces thereof, respectively. The clamp ends 25 include a protrusion 27 in their respective inner surfaces (the surfaces to be contacted with the support portion 24). The protrusions 27 are inserted into respective pivot holes 26. Thus, the earpiece 21 can be openably closed or pivoted about the axis X'--X' passing through the pivot holes 26 and the protrusions 27, as shown by the arrow mark in FIG. 6.
When the earpiece 21 is assembled to the frame 20, it is necessary for an operator, manually or using an appropriate jig, to widen the vertical distance between the clamp ends 25, so as to place the protrusions 27 fitted within the pivot holes 28 in the pivotal support portion 24. To this end, the earpiece 21 is usually formed, at the end thereof, with a notch 28, so that the clamp ends may be easily widened. It is noted, however, that the clamp ends 25 would not withstand a force which tends to urge the protrusions 27 out of the pivot holes 28, when a user intends to wear glasses by opening the earpieces 21, thus causing dislodgement of the protrusions 27 from the pivot holes 26. Same inconvenience may occur, when extraordinary external force, such as axial or torsional force or impact is applied to the earpieces 21.